One conventional injector for injecting fuel into an engine of a vehicle is electro-magnetically operated to open in response to supply of a current to a coil. In an injector driving apparatus for driving multiple injectors, a selection switch is provided at a low-side (low-potential side) of a coil of each injector to select a coil (injector to be driven), to which a current is supplied. Thus, the current is supplied to only the coil corresponding to the selection switch, which is turned on, among multiple switches. In a case that the multiple injectors are not driven for fuel injection at the same time, a current detection element is shared to detect the current supplied to each coil. Specifically, a resistor is provided as the current detection element in a current supply path, through which the current flows to the coils in common (for example, JP 2007-205249 A).
In an engine control system for a vehicle, a power output of the engine is limited when an abnormality arises. One proposal is to provide an injector driving apparatus with a current supply period guard function, which limits a time period of current supply to a coil of an injector to a predetermined period. With the limited period of current supply of the coil, a quantity of fuel injected from the injector is limited and hence the power output of the engine is limited. The current supply period guard function specifically measures a period of continuous flow of the current in the coil and, when the measured period reaches a predetermined period, forcibly stops the current supply to the coil.
If a current is simply supplied to the coil of the injector to diagnose whether the current supply period guard function is normal or not, the injector is driven to inject fuel unnecessarily.